The invention relates to a process for the production of hollow articles having fluorinated inside surfaces consisting of thermoplastic materials. In addition to HDPE, also IDPE, PP, EPDM, PVC and similar materials are suitable as thermoplastic materials.
In the automobile industry, fuel containers are increasingly produced from polymers, in particular, from high pressure polyethylene. These plastic fuel containers are lighter and cheaper than the conventional containers made from sheet steel.
Moreover, they can be made without problems in a complicated shape so that the available space can be better utilized and the container volume can be increased.
Plastic containers are, however, not completely tight for solvent motor fuel and readily volatile or gaseous substances since slight amounts continuously escape to the outside as a result of permeation. Recently, an effort was, therefore, made to considerably reduce the permeation rates. This can be successfully accomplished by exposing the inside surface of the container for some time to the influence of a fluorine-containing treatment gas. During this process, the surface is coated by elementary fluorine or also in the form of fluorocarbon compounds and fluorocarbonhydrogen compounds. Suitable reaction agents are also chlorofluoride, chlorotrifluoride, bromotrifluoride, fluorosulfonic acid and similar substances. The fluorine-containing layer considerably reduces the permeation rates. When the fluorine-containing treatment gas is used at the same time as blow medium in blow extrusion of containers, we speak of in-linefluorination. On the other hand, the fluorination conducted on already extruded containers is called off-line-fluorination. Such processes are known, for example, from German Patent No. 2 401 948 and German Patent No. 2 644 508. In addition to the in-line single step process according to German Patent No. 2 401 948, German Patent No. 3 523 137 also describes an in-line two step process.
The fluorination of plastic surfaces, however, not only influences the permeation behavior to a large degree but also the abrasion resistance, the chemical, thermal and mechanical strength, the adhesive behavior and the wettability are affected. The invention is, therefore, not only limited to the fluorination of the inside surface of plastic fuel containers. In the fluorination, the polymer surface is exposed to an attack by elementary fluorine. In the simplest case, for example, with polyethylene, an incremental radicalic substitution of the CH-bonds by CF-bonds takes place.
Extensive in-house investigations have shown that, depending on the reaction conditions, very differently structured fluorinated layers are produced. In order to attain certain good and clearly reproducible surface effects for the above cited material behavior, the exact adherence to certain structure parameters of these fluorinate surfaces is extremely important. These are in the first place layer thickness, uniformity of the fluorine coating, distribution of the CH2-groups, CHF-groups and CF2-groups and the depth profile.